The present invention is concerned with an apparatus and method for sparging or scavenging molten metal by injection of gas. The invention is primarily directed to the treatment of aluminium and its alloys, but is also useful for the treatment of other non-ferrous metals (and their alloys) such as copper, tin, zinc, lead, magnesium and brass.
It has long been known to reduce the gas content of molten metal and/or to remove dissolved volatile metallic impurities and/or to remove solid or liquid inclusions by passing a stream of gas bubbles through molten metal in a transfer ladle or a holding furnace before supply to a casting station or in transit from the holding furnace to the casting station. In general it is preferred to carry out a sparging or scavenging treatment as close as possible to the casting station so as far as possible to avoid recontamination of the molten metal before casting. However in many circumstances the sparging of the molten metal in the furnace is more convenient.
While the apparatus of the present invention has been designed to meet a problem which is common to both in-furnace sparging and in-transit sparging, it is intended in particular to simplify the performance of sparging in transit by transfer trough from the holding furnace to the casting station.
It is already known to treat molten metal while in transit from the holding furnace to the casting station by a variety of techniques, some of which involve gas injection, possibly in conjunction with filtration, while some involve filtration alone. The known treatment methods under consideration all require the passage of the molten metal through a special treatment station, which frequently requires separate heating to maintain a separate body of metal in a molten condition in a holding bath. The amount of metal so maintained may be as much as 1500 kg. or even 4000 kg. or more depending on the type of apparatus used. The use of apparatus of this kind is open to the objection that it occupies valuable floor space between the holding furnace and the casting station. Moreover, the volume of metal retained in the box must be either drained or flushed each time whenever a different alloy is to be cast, with consequent delay and loss in production. There is also the possibility of some deterioration of the metal during the time which elapses between casting operations. For example, a molten Al-Mg alloy may lose excessive amounts of Mg by oxidation in the holding bath during this period.
It is a principal object of the invention to increase efficiency of sparging by generating a fine dispersion of bubbles of sparging gas in the molten metal.
It is a further object of the invention to provide an improved apparatus for injection of gases into molten metal of such construction that the molten metal may be treated in the course of transit through a normal trough lying between the holding furnace and the casting station.
The efficiency of a mass of gas in scavenging gaseous and other impurities from molten metal is a function of the total surface area of the gas bubbles in contact with the melt at a given time as well as of the distribution and spacing of the bubbles through the melt. In general it may be said that the gas bubbles should be as small as is practicable provided they are not so small that the metal solidifies before they have risen to the surface. If this happens, the gas bubbles become entrapped in the cast ingot, causing micro-porosity. In most gas-sparging operations the gas is injected either through a open-ended lance or through a gas-permeable porous plate, which itself may form part of a lance. As compared with other liquids the interfacial tension at a gas/molten metal interface is very high with the result that at any surface through which gas is being emitted there is a tendency for the incipient gas bubble to spread out sideways on the non-wetted surface. At a porous plug, for example, this phenomenon can lead to agglomeration of incipient bubbles into a single large bubble, which floats up through the molten metal and is relatively ineffective, in terms of gas usage, because of its low surface area/volume ratio. Somewhat similar results occur when a conventional open-ended lance is substituted by a close-ended lance having a plurality of apertures in its side walls. It is also known to use rotating impellers to break up gas bubbles after they have separated from a lance or porous plate. However the latter solution may be inconvenient because it requires the use of a separate treatment station with the attendant inconveniences, already explained above, and because it may cause microporosity problems due to the carry-over in the molten metal of some exceedingly fine gas bubbles which are inevitably generated by the process.